A new sensor design that can, in principle, read out two different physical quantities at the same time has been proposed and analyzed entirely through numerical simulation. The device is a dual-core, D-shaped photonic crystal fiber that exploits surface plasmon resonance to detect both the refractive index of a surrounding liquid or gas and the ambient temperature over a single, combined measurement. According to the design study, published in Applied Physics B, the optimized configuration covers a refractive index range of 1.18 to 1.34 and a temperature span from −40 °C to 100 °C, reaching a maximum refractive index sensitivity of 19,000 nm/RIU at a refractive index of 1.34 and an average temperature sensitivity of −0.5 nm/°C.
The design is aimed at a persistent gap in optical sensing. Many existing dual-parameter fiber sensors concentrate on aqueous samples with refractive indices above 1.33, leaving the interval from 1.0 to 1.33 — the territory of gases and many organic solvents — poorly covered. The authors also point to cross-sensitivity as a chronic problem: temperature fluctuations can shift the refractive index of both the fiber and the analyte, corrupting refractive index readings, while refractive index changes can contaminate temperature measurements. Some designs described as dual-parameter sensors in fact rely on time-division or switching measurements rather than true simultaneous readout.
The proposed structure answers these issues with geometry. Two identical cores sit in a hexagonal lattice of air holes, all of uniform diameter, and two open-ring channels are cut into the D-shaped polished surface of the fiber. The left channel, Channel 1, is coated with a gold film and is dedicated to refractive index sensing. The right channel, Channel 2, is coated with gold and filled with polydimethylsiloxane, or PDMS, a silicone polymer with a high thermo-optic coefficient, and is used mainly for temperature sensing. Because the two channels are physically separated and Channel 1 is insensitive to temperature variations, the design suppresses cross-sensitivity between the two measured quantities while keeping the fiber geometry simple enough to manufacture.
The choice of PDMS is central to the temperature channel. The material is chemically stable, resists corrosion by acids, alkalis and most organic solvents at room temperature, and its physicochemical properties remain essentially unchanged from −50 °C to 200 °C. Its refractive index follows a simple linear relation with temperature, decreasing by 4.5 × 10⁻⁴ per degree Celsius, which is what allows the resonance wavelength in Channel 2 to track temperature so cleanly. Gold is used in both channels because of its chemical stability and oxidation resistance, standard reasons for preferring it over silver in surface plasmon resonance devices.
The entire analysis was carried out with the finite element method, using a mesh of 19,870 domain cells and 1,446 boundary cells, with a perfect matching layer as the outer boundary. The researchers systematically optimized four structural parameters by examining how each affects the loss spectra and three evaluation criteria: the spacing between the two resonance peaks, the total loss of the two peaks, and the total sharpness of the peaks. The gold film thickness was varied from 35 to 55 nm; thicker films shift the resonance wavelength to shorter wavelengths and reduce the loss peak, because less evanescent-wave energy penetrates to the metal boundary. A thickness of 45 nm was selected as offering the most balanced and stable performance.
The open-ring diameter was scanned from 1.6 to 2.0 μm, with larger diameters producing a blue-shift of the resonance wavelength and lower peak losses; 1.8 μm was chosen. The air hole diameter was fixed at 1.6 μm, a value that changes the resonance wavelength only minimally but flattens the resonance peaks at larger sizes. The air hole spacing was set at 2.5 μm, which showed a slight blue-shift and reduced peak loss as it increased. The final parameter set — open-ring diameter 1.8 μm, air hole diameter 1.6 μm, pitch 2.5 μm and gold film thickness 45 nm — was then used for all performance evaluations.
In the optimized configuration, the two channels behave in cleanly separated ways. When temperature rises at a fixed refractive index of 1.25, the loss curve of Channel 1 does not change at all, while Channel 2 shows a rising loss peak and a blue-shifting resonance wavelength. When the refractive index increases at constant temperature, the resonance wavelength in both channels shifts to longer wavelengths with a gradually increasing loss peak. At the coupling points computed for a refractive index of 1.25 and 20 °C, the core mode and surface plasmon mode intersections for the two channels occur at 1.5 μm and 1.615 μm, a difference of about 115 nm that makes the two sensing channels easy to distinguish spectrally.
The reported sensitivity figures break down by channel. Channel 1 achieves a maximum refractive index sensitivity of 11,000 nm/RIU, while Channel 2 reaches a maximum of 19,000 nm/RIU at a refractive index of 1.34 and a maximum temperature sensitivity of −0.5 nm/°C. Amplitude-based detection offers an alternative readout: Channel 1 reaches a maximum amplitude sensitivity of 209.53 RIU⁻¹ and Channel 2 reaches 178.11 RIU⁻¹, with Channel 2 also attaining a maximum temperature amplitude sensitivity of 8.1 × 10⁻³ °C⁻¹. When the refractive index is varied, the sensor exhibits maximum figures of merit of 74 RIU⁻¹ and 99 RIU⁻¹ for the two channels, and under temperature variation Channel 2 reaches a maximum figure of merit of −0.0033 °C⁻¹ at 80 °C.
Converting raw resonance shifts into accurate two-parameter readouts requires decoupling, and the study applies a sensitivity matrix method combined with segmented fitting. Because Channel 1 does not respond to temperature, only Channel 2 needs decoupling when temperature changes. Fitting the whole refractive index range with a single straight line yields poor linearity, so the range 1.18–1.33 is divided into three segments — 1.18–1.22, 1.23–1.29 and 1.30–1.33 — with separate linear fits for each channel. The segment fits reach coefficients of determination between 0.9747 and 1, and the temperature fit for Channel 2, RW = −0.5T + 1630 nm, is linear across the full −40 to 100 °C range with a coefficient of determination of 1, so no segmentation is needed there. The resulting demodulation matrix inverts the channel sensitivities to recover both the refractive index change and the temperature change simultaneously.
Two caveats frame the result. First, this is a numerical design study; no sensor was fabricated, and the reported ranges and sensitivities are simulation outputs from finite element modeling. Second, the authors themselves assessed how fabrication errors would affect the device by varying every optimized structural parameter by ±10%. Within that tolerance, the peak loss changes according to size — larger structures correspond to lower loss — but the resonance wavelength does not change at all, which the authors present as evidence of robustness. They also outline a plausible manufacturing route: silica rods drilled and drawn into capillaries, stacked into a preform, drawn into fiber, side-polished or etched to create the platform and open rings, gold deposited by chemical vapor deposition or sputtering, and PDMS encapsulated by femtosecond laser micromachining. Compared with previously reported dual-parameter sensors — whose listed maxima include a refractive index sensitivity of 34,600 nm/RIU and a temperature sensitivity of 21.5 nm/°C, but with limited low-temperature capability — the proposed design claims competitive advantages in low-refractive-index coverage, its broad temperature range that extends well below room temperature, and structural simplicity, with suggested applications in gas sensing, chemical analysis, biomedicine and low-temperature industrial monitoring.
Subject of Research: A numerically designed dual-core D-shaped photonic crystal fiber surface plasmon resonance sensor for simultaneous refractive index and temperature detection.
Article Title: Dual-core wide detection range D-shaped photonic crystal fiber dual parameters sensor based on surface plasmon resonance
Article References: Tian, J., Du, Z., & Dong, X. (2026). Dual-core wide detection range D-shaped photonic crystal fiber dual parameters sensor based on surface plasmon resonance. Applied Physics B, 132(10), Article 108. https://doi.org/10.1007/s00340-026-08723-z
Image Credits: AI Generated
DOI: 10.1007/s00340-026-08723-z
Keywords: photonic crystal fiber, surface plasmon resonance, refractive index sensing, temperature sensing, dual-parameter sensor, PDMS, gold film, finite element method, decoupling, low refractive index, sensor design, simulation study
News Source: Denise Maddox. (October 28, 2026). Dual-Core Fiber Sensor Design Simultaneously Tracks Refractive Index and Temperature. Scienmag.
